1. Field of the Invention
The present invention relates to a high-temperature superconductor that has very low superconducting anisotropy, a high superconducting transition temperature (Tc), a high critical current density (Jc), a high irreversible field (Hirr), and a long coherence length "xgr" in a direction perpendicular to the plane (the direction being the c-axis direction, the intrafacial plane the ab-axis), and to a method for producing the superconductor.
2. Description of the Prior Art
A high Tc has been considered to be closely related to high superconducting anisotropy (two-dimensionality) of superconducting properties. Known high-temperature superconductors having a two-dimensional layered structure comprised of charge reservoir layers and superconducting layers include Y, Bi, Tl, and Hg based copper oxide superconductors. However, owing to the high superconducting isotropy of these superconductors, at 77 K they do not have a sufficiently high superconductivity, impeding progress toward their practical application at liquid nitrogen temperatures.
In existing superconductors having a layered structure, owing to the large radius of the Ca ions in the CunCanxe2x88x921O2n of the superconducting layers the superconductive coupling between the CuO2 layers is low. Moreover, because in these superconductors having a layered structure the charge reservoir layer is an insulation layer or non-superconducting layer with low superconductive coupling in the c-axis direction and, therefore, the interactive effect between superconducting layers is small, in addition to which owing to the thinness of the superconducting layers, superconducting anisotropy xcex3 is large, being in the order of 5 to 300 (xcex3 defined as the ratio of the coherence length, the square root of the electron effective mass ratio, or magnetic field penetration depth ratio, is xcex3="xgr"ab/"xgr"c=(mc/mab) xc2xd=xcexc/xcexab).
As such, the Jc, especially the Jc under a high magnetic field, and the Hirr, the upper limit of the magnetic field at which zero electrical resistance is generates, become small, posing many problems to the practical use as wire or bulk superconducting material. Furthermore, the large superconducting anisotropy means that the (Jc)c in the c-axis direction is small and the coherence length "xgr"c in the c-axis direction also is small, so that when used as a superconducting device material, the properties of the layered structure superconducting device are not adequate, especially the Josephson current density.
An object of the present invention is to provide a high-temperature superconductor that is low in superconducting anisotropy, and a method for producing the superconductor.
In accordance with the present invention, the above object is attained by providing a Mg-doped high-temperature superconductor having low superconducting anisotropy, comprising a two-dimensional layered structure constituted by a charge reservoir layer and a superconducting layer, wherein some or all atoms comprising the charge reservoir layer are Cu, O atoms, metallizing or rendering the charge reservoir layer superconducting, a portion of Ca of CunCanxe2x88x921O2n constituting the superconducting layer is replaced by Mg, increasing superconductive coupling between CuO2 layeres, a thickness of the superconducting layer is increased, and therefore coherence length in a thickness direction is increased based on the uncertainty principle, lowering superconducting anisotropy.
The Mg-doped high-temperature superconductor having low superconducting anisotropy may be produced by a method comprising supplying a superconductor comprising Cu1xe2x88x92xMx(Ba1xe2x88x92ySry)2(Ca1xe2x88x92zMgz)nxe2x88x921CunO2n+4xe2x88x92w (in which M is one or more selected from the group consisting of Tl, Hg, Bi, Pb, Au, In, Sn, Mg, Ag, Mo, Re, Os, Cr, V, Fe, and lanthanide elements, 0xe2x89xa6x less than 1, 0xe2x89xa6y less than 1, 0xe2x89xa6z less than 1, 0xe2x89xa6wxe2x89xa64, and 3xe2x89xa6nxe2x89xa616), or Cu1xe2x88x92xMx(Ba1xe2x88x92yxe2x88x92mSryRm)2(Ca1xe2x88x92zMgz)nxe2x88x921CunO2n+4xe2x88x92w (in which M is one or more selected from the group consisting of Tl, Hg, Bi, Pb, Au, In, Sn, Mg, Ag, Mo, Re, Os, Cr, V, Fe, and lanthanide elements, R is one or more selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, with 0 less than mxe2x89xa61, 0xe2x89xa6y+mxe2x89xa61, 0xe2x89xa6x less than 1, 0xe2x89xa6y less than 1, 0 less than z less than 1, 0xe2x89xa6wxe2x89xa64, and 3xe2x89xa6nxe2x89xa616) or the starting materials thereof onto a single-crystal substrate or crystal-oriented substrate, sealing the substrate in an oxidation resistant capsule and applying a pressure of at least one atmosphere to synthesize bulk or single-crystal superconducting materials having a high critical current density Jc aligned along at least the a-axis and c-axis.
The Mg-doped high-temperature superconductor having low superconducting anisotropy according to the present invention may also be produced by a method comprising depositing or applying the above superconductor or superconductor starting material on a single-crystal substrate or crystal-oriented film substrate, sealing them in an oxidation resistant capsule to obtain a single-crystal or crystal oriented film aligned at least along the a-axis and c-axis having a high critical current density Jc.
Thus, in the superconductor of this invention, a portion of the Ca of the CunCanxe2x88x921O2n constituting the superconducting layer is replaced by Mg, which has a small ion radius, thereby increasing the superconductive coupling between the CuO2 layers, a thickness of the superconducting layer is increased, and therefore superconducting anisotropy is lowered based on the uncertainty principle.
The above and other features of the present invention will become apparent from the following description made with reference to the drawings.